BGL气化炉新型建模方法及优化分析

Numerical simulation and optimization analysis of BGL gasifier with a novel model

充分考虑碎煤加压熔渣气化(British Gas Lurgi, BGL)炉内的流动特性,将BGL气化炉分为并流段和逆流段,并依此建立了BGL气化炉的一维稳态机理模型.并利用该模型模拟了以烟煤为原料的某工业BGL气化炉,模拟结果与工业实测值误差小于2%,从而验证该模型的正确性.然后在所得到模型的基础上探讨了氧煤比及汽煤比对气化性能的影响.研究表明,氧煤比对碳转化率以及蒸汽分解率的影响较大,随着氧煤比增加,碳转化率增大,蒸汽分解率先增大后减小,当氧煤比为0.47时,蒸汽分解率最大;汽煤比增大有利于水煤气变换反应,但该反应受化学平衡限制,当气化剂充足时,蒸汽的消耗量将小于其增加量,随着汽煤比增大蒸汽分解率逐渐降低.

Coal is utilized by combustion has resulted in various pollutants,especially the emission of greenhouse gases(such as CO2)that may contribute to climate change.Hence,CO2 reduction has become an urgent task for clean coal utilization.Coal gasification is one of the most efficient technology to utilize coal in the production of various chemicals,fuels,and electricity with less emission of CO2.There are three types of advanced coal gasification technologies,namely fixed-bed gasifier(Lurgi gasification technology),entrained-flow gasifier(Texaco,opposite multi-burner,and Shell gasification technologies),and fluidized-bed gasifier.Lurgi gasification technology,such as British Gas Lurgi(BGL),is improved from the traditional Lurgi gasifier and has demonstrated to be reliable and beneficial for low-rank coal conversion.Compared with the traditional Lurgi gasifier,slag is discharged as molten state in the BGL gasifier because the reaction temperature is higher than the ash fusion point of the coal.In this condition,it promotes carbon conversion and steam decomposition.In addition,it produces less tar and phenolic compounds than the traditional Lurgi gasifier because of its very high operating temperature.However,the composition profile and the temperature inside the reactor are extremely difficult to measure due to elevated operating temperature and pressure.Therefore,an accurate simulation and quantitative analysis of the BGL gasifier is very important for improving the scientific understanding of the gasifier operating performance and facilitating the optimization of the gasifier to achieve higher gasifier efficiencies.In the BGL gasifier,coal is fed from the top of the gasifier and moves downward by gravity.Due to the higher temperature at the bottom of the gasifier,ash is melted and liquid slag is formed.Gasification agent(O2 and steam)is injected into the BGL gasifier through inclined downward nozzles.In the zone below the nozzles,the gasification agent flows downwards concurrent with coal particles and participates in reactions.Then the unreacted gasification agent and gas products flow upwards countercurrent with coal particles after reaching the slag interface.In the zone above the nozzles,the gas-solid phase maintains countercurrent flow.The fluid flow in the zone below the nozzles is complicated.To simplify the process,the flow pattern in this zone is treated as the gas-solid phase cocurrent flow.According to the fluid flow characteristics,a one-dimensional steady-state model of the BGL gasifier was established.In this model,the BGL gasifier was divided into the concurrent-flow and the countercurrent-flow zones.An industrial BGL gasifier was simulated by the proposed model and the simulation results were compared with its operation data.It was shown that the error between the simulation results and the operation data was less than 2%.Furthermore,the effects of oxygen-to-coal ratio and steam-tocoal ratio on the performance of the gasifier were investigated.The results indicated that carbon conversion and steam decomposition were influenced by different oxygen-to-coal ratio.As oxygen-to-coal ratio increases,carbon conversion increases gradually but steam decomposition increases firstly and then decreases.When oxygen-to-coal ratio is 0.47,the maximum of steam decomposition is obtained.Higher steam-to-coal ratio is beneficial to the water-gas shift reaction.The reason is that this reaction is restricted by chemical equilibrium.When the gasification agent is sufficient,the consumption of steam can be ignored than the increase of the steam in the feedstock.Therefore,the steam decomposition decreases gradually with the increase of the steam-to-coal ratio.According to the simulation results,the optimal operating conditions of the BGL gasifier are obtained,that is,oxygen-to-coal ratio is 0.47-0.49 and steam-to-coal ratio is 0.32-0.40.